Riedel thyroiditis (RT) is an uncommon form of chronic thyroiditis in which the thyroid gland is replaced by fibrous tissue. The etiologic mechanisms underlying RT are unclear: the prevailing view is that it is part of a generalized fibroinflammatory process also involving other organs. The clinical manifestations of RT are protean, often resembling malignancy owing to goiter of remarkably hard consistency. Physical examination, laboratory analysis, cytology, and imaging features are not useful for differentiating between RT and neoplastic diseases or the fibrous variant of Hashimoto thyroiditis in the presurgical evaluation of patients. Histologic examination is necessary to establish the final diagnosis of RT. The present article reviews the most recent concepts about etiologic mechanisms, pathogenesis, diagnosis, and management of RT